The present embodiments relate to variable input impedance preamplifiers. For example, a variable input impedance preamplifier is used for ultrasound imaging. Other uses may be provided.
For ultrasound imaging, the transducer preamplifier is selected based on a trade-off of input impedance to get as close as possible to desired performance. A single impedance amplifier is typically used. Amplifier impedance and dynamic range are traded with only one optimized for the input signal. In ultrasound, the signal strength varies over time or as a function of depth from which received echo signals are reflected.
Ultrasound transducers have low impedance at a resonant frequency and higher impedance away from resonance. There may be less loss of signals away from resonance with a high impedance amplifier than with a low impedance amplifier due to the dominance of the transducer source impedance in the resistive divider formed between the transducer impedance and the preamplifier input impedance. This lower loss may enhance signal-to-noise ratio and bandwidth. With a low impedance preamplifier, the resistive divider is dominated by the preamplifier, causing higher loss of signals away from resonance and resulting in lower signal-to-noise and reduced bandwidth. The impedance of the transmit/receive switch may also causes higher losses with lower impedance preamplifiers.
Some amplifiers with variable input impedance have been used in ultrasound. For example, a switched resistance in a feedback path of a fixed gain amplifier is used to control gain, but also varies input impedance. Such a switched amplifier topology may change the impedance, but with an interruption in operation for switching.